1. Field of the Invention
The present invention relates to an electric ignition unit for igniting propellants. The propellants can be solid, liquid or gaseous. The important components of such an ignition unit are at least a pin made of an electrically conductive material, a pin holder comprising a metal ring which is overdimensioned relative to the pin and thus encloses an annular space with the same, as well as an electrically insulating filling in the annular space. At the one end of the pin holder there is a sealing cap for receiving the propellant (igniting agent). At the other end of the pin holder there is a plug which is connected with the pin holder in a random manner and immerses into the pin during operation in a contacting fashion and thus produces an electrically conductive connection. The entire apparatus is situated in a housing.
The filling of the pin holder generally consists of an insulating material, e.g. of glass. Such a material is molten in the metal in a vacuum-tight manner.
2. Description of the Related Art
Such ignition units are widely used in electronics and electrical engineering. The glass used for melting is used in this case as an insulator. Typical glass-to-metal seals are configured as follows: metallic internal conductors are molten into a preformed sintered glass element, with the sintered glass element being molten into an outer metal part.
The said ignition units are used for airbags or seat-belt tightening systems in motor vehicles. The entire ignition unit with the pin holder configured as a glass-to-metal seal comprises an ignition bridge. The glass-to-metal seal plays an important role. It is necessary in order to supply to a housing in a reliable and insulated manner the electrical voltage which occurs through one or two metallic pins.
Known glass-to-metal seals are configured as follows: a glazed or insulated pin and an additional conductor as a ground wire are used; the ground wire is usually welded or soldered. It thus produces the necessary connection to the metallic housing. This embodiment is very complex and expensive to produce. A serious disadvantage is that the ground connection to the external housing is relatively insecure. This can have serious consequences, especially in the case of the mentioned applications of the airbag and the seat-belt tightening system.
Known ignition units of the aforementioned or similar kind have been described in U.S. Pat. No. 6,274,252, U.S. Pat. No. 5,621,183, DE 29 04174 A1 or DE 19927233 A1.
All the said ignition units comprise two metal pins. The present invention deals especially with electric ignition units which have only one pin.
Serious problems of the aforementioned ignition units are the following:
The transitions between the conductive parts have a resistance which is relatively high. A further disadvantage of known ignition units is the service life. This is unsatisfactory in known ignition units. This is caused at least in part by corrosion of the involved components.